Problems with damp can be very difficult to diagnose, especially without seeing the property.
It could be caused by penetrating damp, where the moisture is soaking THROUGH the wall either because the wall is solid (as in many older properties, your case, possibly?) or as mentioned by Carlton, the wall ties in a cavity wall are allowing moisture to seep through due to mortar droppings caused by shoddy workmanship.
Or it could be RISING damp, where the DPC is non-existent, has failed due to age or has been breached by, for example, earth piled up against the wall or paving being laid too high (should be 150mm below DPC).
Penetrating damp often has an associated cause, such as a damaged rainwater pipe allowing water to discharge onto brickwork saturating it over time.
Either way, like I said in my previous post, the best thing to do is have somebody survey the property and make a proper diagnosis. Get a reputable company, (ask friends for recommendations if you don't know any) and then take their advice. It may be costly but what is the alternative?